Building on last week’s post on Phylogenies, this week I’ll introduce the concept of “tree thinking.” This short discussion is based on the paper “The Tree-Thinking Challenge” by Baum et al. (2005) Science 310:979-980.
Below are two phylogenetic trees illustrating the evolutionary relationships among four groups of species: fish, amphibians, reptiles, and mammals (mice and humans). Keep in mind that this is a very simple depiction of the tree of life that focuses only only a few groups. So, what do these trees mean?
To find out, you first need to know how to read a phylogeny. Unlike books, you don’t read a phylogeny right-to-left and top-to-bottom. Instead, you read them bottom-up, which is akin to traveling back in evolutionary time. At the top of the tree (the leaves), we are in the present. As we go down the tree, we travel into the evolutionary past. Also, you should concentrate on the nodes, or those forks in the tree that indicate evolutionary branching points (see the points labeled x and y on the tree to the left). For example, starting with the tree on the left, you can see there are four nodes (two of them are labeled x and y). These nodes denote clades, or groups of species that evolved from common ancestor long ago. Remember that, ultimately, all living species share a common ancestor, but that some species share a common ancestor more recently than others. That’s what we see here: species joined by node x share a common ancestor more recently than species joined by node y. Hence, node y is farther back on the tree than node x.
Now, enough with boring x and y symbols. What this tree means is that the five species are joined into four clades, one for each node. These clades are:  fish-amphibians-reptiles-mammals (all vertebrates, or animals with backbones),  amphibians-reptiles-mammals (all tetrapods, or vertebrates with four limbs),  reptiles-mammals (all amniotes, or tetrapods with watertight eggs), and  mice-humans (both mammals, or amniotes with fur and mammary glands). The interpretation of this tree is that mice and humans shared a common ancestor more recently than did humans and lizards, or humans and fish. Interestingly, the tree to the right shows the exact same relationship, but the branches have been turned around on their nodes. (See for yourself, reading the tree on the right bottom-up.) This illustrates the danger of interpreting trees along the top from left-to-right.
Another point that these trees illustrate is that, contrary to what many creationists say or think, humans did not evolve from mice. Nor did humans evolve from reptiles, amphibians or fish. This tree illustrates that humans shared a common ancestor with each of these species. Put another way, at one point in time a vertebrate swam the ocean; its population was split and evolved into what we recognize today as two groups: fish, and tetrapods. This process of splitting (called speciation) occurred again along the tetrapod lineage: an ancestral tetrapod population was split, and each group evolved new traits ultimately giving rise to living amphibians and amniotes. What happened next? You guessed it: speciation, leading to groups that became reptiles and mammals.
Why should you care about any of this? You should care because phylogenies are the road-map to evolution, which has practical implications for understanding human diversity and disease. For example, did you know that phylogenies of the HIV/AIDS virus were used to discover the year the first outbreak of HIV occurred, which country it occurred in, and identify how it was transmitted? And did you know that phylogenies of the HIV virus have been presented as evidence in court to convict a doctor of intentionally infecting others with the virus? Similar studies have been done to track major outbreaks of other viruses, including bird flu. Hence, there are good reasons to bone up on your tree-thinking abilities. If you’ve got time to kill and want practice, you can find some simple (and some not-so-simple) exercises here.
Well, I’ve reached my word limit. If you have any questions, comments, or corrections, feel free to let me know in the comments below.